Casting some spells requires particular objects, specified in parentheses in the component entry. A character can use a component pouch or a spellcasting focus (found in “Equipment”) in place of the components specified for a spell. But if a cost is indicated for a component, a character must have that specific component before he or she can cast the spell.

Does requiring "a copper piece" mean that a "cost is indicated" and so it requires one to actually be holding a coin, rather than just using a spellcasting focus or component pouch?

Usually a cost being indicated is worded like "something worth at least 25 gp", so since this isn't phrased as "a copper piece worth 1 cp" or even "a one-cp coin", I'm not sure that it means that a "cost is indicated". It seems to more just be describing a random object associated with the spell like other "normal" spell material components. But then again, I can see a good argument that a coin used for currency would be the most straightforward way of indicating a cost that there is.

Obviously in general adventurers won't usually have a lot of trouble finding a copper piece during their travels, but I could imagine some circumstances where it would be inconvenient for a spellcaster to need to dig a coin out of their pouch while their spellcasting focus is already in hand.

3 Answers
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"Penny for Your Thoughts"

Although the writers were almost certainly referring to a common copper coin for their joke, there's no indication this was meant as an actual "cost". The 5e authors consistently use the term "worth" or "value" whenever an actual material component cost is indicated, and they express that cost in gp (I've yet to find one costing less than 5 gp).

Copper pieces are so low in value that I don't believe anyone would consider them a real "cost" (even if it was say, 11 copper pieces to cast Gentle Repose on a Beholder). They're less valuable and far easier to find than most cost-free material components.

I wouldn't spend much time worrying about it.... but that's just my 2 cp.

No - "A copper piece" does not indicate a cost.

Not all pieces of copper are copper coins worth 1 cp. We aren't told anything about the value of the copper piece required, therefore the spell requires a generic copper piece (or piece of copper). When looking at spells like Augury and Identify, we see the format used to indicate a cost (which is absent in Detect Thoughts).

Any old copper piece will do, as would a spellcasting focus or component pouch.

\$\begingroup\$Isn't "copper piece" the actual name of the coinage? Like "gold piece" and "silver piece"?\$\endgroup\$
– Michael RichardsonMay 17 '18 at 17:14

\$\begingroup\$I think to help clarify this, it should be considered a piece of copper rather than a copper piece. If named that way, the issue of the literal meaning and the usage of the term for the currency will not be mixed up\$\endgroup\$
– DMateMay 17 '18 at 17:17

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\$\begingroup\$And it's precisely that it's phrased the way is, as "a copper piece" (to make the pun), that makes me think that it isn't supposed to be "any old scrap of copper".\$\endgroup\$
– user37158May 17 '18 at 22:33

\$\begingroup\$I think the intent is that you use a 1cp piece (for the pun) but that they actively chose not to write the value so that you CAN use any random piece of copper. On all the other\$\endgroup\$
– M CMay 21 '18 at 18:36

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\$\begingroup\$1CP is a sum of money (albeit one that is hard to break down into smaller sums!). Sums of money can be made up of combinations of currency. A copper piece is a specific coin (and "a copper piece" is distinct from "a piece of copper")\$\endgroup\$
– QuentinMay 21 '18 at 21:14

Yes, you need a copper piece

The spell says as its material cost "a copper piece". A copper piece is worth one copper piece. There is value to the material component. Other spells that have a cost are items that are not currency, such as, ruby dust worth 50gp for the continual flame spell. All of these spells that have a cost to it are specifying items that are not currency. That is clearly why it has to say "worth" in the description.

The Detect Thoughts spell does not need to have the wording like those of the other spells because the material is the actual currency. It would be redundant to say "a copper piece worth 1cp". The spell is specific in the wording saying "a copper piece" instead of "a piece of copper".